Movement is produced and coordinated by several interacting brain centers, including the motor cortex, the cerebellum, and a group of structures in the inner portions of the brain called the basal ganglia. Sensory information provides critical input on the current position and movement of body parts and spinal nerve cells (neurons) help prevent opposing muscle groups from contracting at the same time.
Motor cortex: Voluntary (willed) motor commands begin in the motor cortex located on the outer, wrinkled surface of the brain. Movement of the right arm is begun by the left motor cortex, which generates a large volley of signals to the involved muscles. These electrical signals pass along upper motor neurons through the midbrain to the spinal cord. Within the spinal cord, they connect to lower motor neurons that convey the signals out of the spinal cord to the surface of the muscles involved. Electrical stimulation of the muscles causes contraction and the force of contraction pulling on the skeleton causes movement of the arm, hand, and fingers.
Damage to or death of any of the neurons along this path causes weakness or paralysis of the affected muscles.
Antagonistic muscle pairs: Contraction of the biceps muscle, located on the top of the upper arm, pulls on the forearm to flex the elbow and bend the arm. Contraction of the triceps, located on the opposite side, extends the elbow and straightens the arm. Within the spine, these muscles are normally wired so that voluntary contraction of one is automatically accompanied by blocking of the other. In this way, these antagonist muscles are kept from resisting one another. Spinal cord or brain injury can damage this control system and cause involuntary simultaneous contraction and spasticity, an increase in resistance to movement during motion.
Once the movement of the arm is initiated, sensory information is needed to guide the finger to its precise destination. In addition to sight, the most important source of information comes from the "position sense" provided by the many sensory neurons located within the limbs (proprioception). Proprioception is what allows an individual to touch their nose with their finger even with the eyes closed. The balance organs in the ears provide important information about posture. Both postural and proprioceptive information are processed by a structure at the rear of the brain called the cerebellum. The cerebellum sends out electrical signals to modify movements as they progress, organizing the voluntary commands into a tightly controlled pattern. Cerebellar disorders cause ataxia (inability to control the force, fine positioning, and speed of movements). Disorders of the cerebellum may also impair the ability to judge distance so that a person under- or overreaches the target (dysmetria). Tremor during voluntary movements can also result from cerebellar damage.
The basal ganglia:
Both the cerebellum and the motor cortex send information to the basal ganglia (a set of structures deep within the brain that help control involuntary components of movement). The basal ganglia send output messages to the motor cortex, helping to initiate movements, regulate repetitive or patterned movements, and control muscle tone.
Circuits of neurons (nerve cells) within the basal ganglia are very complex. Within the structure, some groups of cells begin the action of other basal ganglia components and some groups of cells block the action. These complicated feedback circuits are not entirely understood. Disruptions of these circuits are known to cause several distinct movement disorders. A portion of the basal ganglia called the substantia nigra sends electrical signals that block output from another structure called the subthalamic nucleus. The subthalamic nucleus sends signals to the globus pallidus, which in turn blocks the thalamic nuclei. Finally, the thalamic nuclei send signals to the motor cortex. The substantia nigra then begins movement and the globus pallidus blocks it.
This complicated circuit can be disrupted at several points. For instance, loss of substantia nigra cells, as in Parkinson's disease, increases blocking of the thalamic nuclei, preventing them from sending signals to the motor cortex. The result is a loss of movement (motor activity), a characteristic of Parkinson's.
In contrast, cell loss in early Huntington's disease decreases blocking of signals from the thalamic nuclei, causing more cortex stimulation and stronger but uncontrolled movements.
Disruptions in other portions of the basal ganglia are thought to cause tics, tremors, dystonia, and a variety of other movement disorders, although the exact mechanisms are not well understood.
Movement disorders can result from causes including age-related changes, environmental toxins, genetic disorders (such as Huntington's disease and Wilson disease), medications (antipsychotic drugs), metabolic disorders (hyperthyroidism and diabetes), Parkinson's disease, and stroke (neurological damage due to lack of blood supply to the brain).
Some movement disorders, including Huntington's disease and inherited ataxias, are caused by inherited genetic defects. Some diseases that cause focal dystonia, or sustained muscle contraction limited to a particular muscle group, are inherited, but others are caused by trauma. The cause of most cases of Parkinson's disease is unknown, although genes have been found for some familial forms.
Diagnosis of movement disorders involves taking a family history, a history of symptoms, and performing a physical examination (including neurological examination) and various tests (including blood tests and imaging tests).
Blood tests: Blood tests
may include a complete blood count (CBC), a creatine kinase test, and a DNA analysis (to determine if the disorder is genetic). In some cases, a cerebrospinal fluid (CSF) analysis also is performed.
Cerebrospinal fluid analysis involves performing a spinal tap or lumbar puncture. In this procedure, about two tablespoons of cerebrospinal fluid is drawn into a needle inserted between two lumbar vertebrae and then examined under a microscope. This procedure is usually performed in a hospital or clinic under local anesthesia, although general anesthesia can be used. Side effects include pain and tenderness in the area of puncture.
Imaging tests: Imaging tests, including computed tomography (CT scan), magnetic resonance imaging (MRI scan), and positron emission tomography (PET scan), may be used to detect damage (such as shrinkage) in the basal ganglia, structural abnormalities, and stroke (neurological damage due to a lack of oxygen to the brain).
Other imaging tests: An electromyogram (EMG) and an electroencephalogram (EEG) also may be performed. These tests are used to monitor electrical activity within the body and can help detect nerve and muscle disorders. EMG involves placing electrodes on the skin (surface EMG) or into the muscle (intramuscular EMG) to record electrical activity of the muscle. In an EEG, electrodes are attached to the scalp and connected to a machine that records electrical impulses in the brain.
Muscle biopsy: A muscle biopsy may also be performed to distinguish between nerve and muscle disorders. This procedure, which is performed under local anesthesia, involves making a small incision and removing a sample of muscle for microscopic evaluation. Following the procedure, patients may experience minor pain and bruising at the biopsy site for about one week.
signs and symptoms
Symptoms of movement disorders often vary and fluctuate. The severity of symptoms may be affected by factors such as anxiety, fatigue, medications, and stress.
Some movement disorders cause hyperkinesia (excessive spontaneous movement or abnormal involuntary movement) and others cause hypokinesia (absent or reduced ability to perform purposeful movement).
Abnormal movements may be rhythmical (essential tremor) or irregular and may be rapid and jerky (tics) or slowed and sustained (Parkinson's disease, dystonia). In most cases, irregular movement cannot be consciously controlled or suppressed.
Akathesia: Akathesia is restlessness and a desire to move to relieve uncomfortable sensations. Sensations may include a feeling of crawling, itching, stretching, or creeping, usually in the legs.
Athetosis. Atheitosis is the slow, writhing, continuous, uncontrollable movement of the arms and legs.
Ballism: Ballism is like chorea, but the movements are much larger, more explosive, and involve more of the arm or leg. This condition, also called ballismus, can occur on both sides of the body or on one side only (hemiballismus).
Chorea: Chorea is rapid, nonrhythmic, usually jerky movements, most often in the arms and legs.
Dystonia: Dystonia is sustained muscle contractions, often causing twisting or repetitive movements and abnormal postures. Dystonia may be limited to one area (focal) or may affect the whole body (general). Focal dystonias may affect the neck (cervical dystonia or torticollis), the face (one-sided or hemifacial spasm, contraction of the eyelid, or blepharospasm, contraction of the mouth and jaw or oromandibular dystonia, simultaneous spasm of the chin and eyelid, or Meige syndrome), the vocal cords (laryngeal dystonia), or the arms and legs (writer's cramp, occupational cramps). Dystonia may be painful as well as incapacitating.
Myoclonus: Myoclonus is a sudden, shock-like muscle contraction. Myoclonic jerks may occur singly or repetitively. Unlike tics, myoclonus cannot be controlled even briefly.
Tics: Tics are involuntary, rapid, nonrhythmic movements or sounds. Tics can be controlled briefly.
Tremor: Tremors are involuntary (uncontrollable) shaking of a body part. Tremors may occur only when muscles are relaxed or they may occur only during an action or while holding an active posture.
Bradykinesia: Bradykinesia is slowness of movement.
Freezing: Freezing is the inability to begin a movement or involuntary stopping of a movement before it is completed.
Postural instability: Postural instability is the loss of ability to maintain upright posture caused by slow or absent righting reflexes.
Rigidity: Rigidity is an increase in muscle tension when an arm or leg is moved by an outside force.
Complications of movement disorders depend on the individual and the particular condition that is present.
Mental and emotional problems: As well as movement difficulties, there are a variety of behavioral and psychological difficulties that are experienced by many, though not all, individuals with movement disorders. The most frequently reported behavioral problems are attention deficits, obsessions, compulsions, impulsivity, irritability, aggression, immaturity, drug and alcohol abuse and addiction, self-injurious behaviors, and depression. Some individuals with movement disorders (such as Tourette's syndrome) have significant problems with labile (rapidly changing) emotions, impulsivity, and aggression directed to others. Temper fits, include screaming, punching holes in walls, threatening others, hitting, biting, and kicking, are common in such patients.
Along with psychiatric conditions, individuals with movement disorders, such as Huntington's disease, may be more susceptible to suicidal thoughts.
Other health complications: Some movement disorders, such as Huntington's disease, can make the individuals susceptible to life-threatening complications related to this disease, such as pneumonia (a bacterial infection in the lungs and respiratory system) and heart disease (including heart attack, congestive heart failure or CHF, and atherosclerosis or hardening of the arteries) are the two leading causes of death for individuals with Huntington's disease. Additionally, Huntington's disease patients have higher incidence of choking and respiratory complications, gastrointestinal diseases (such as cancer of the pancreas), and suicide than the population without the condition. Ocular (eye) complications (including nystagmus or involuntary eye movements) may be present in some individuals with movement disorders, including Wernicke-Korsakoff syndrome (results from thiamin or vitamin B1 deficiency).
Age: Parkinson's disease affects about one percent of people older than 65 and 0.4% of those older than 40. The mean age at onset is about 57 years of age. Rarely, Parkinson's disease begins in childhood or adolescence (juvenile parkinsonism). Tourette's syndrome usually becomes apparent in children between ages two and 15, with approximately 50% of patients affected by age seven. The age of symptom onset is typically before the age of 18.
Genetics: Some movement disorders are due to inheriting genes that carry the genetic condition. One or both parents can have the gene and pass it on to their children. Some individuals may carry the gene but not exhibit symptoms of the neurological condition. Examples of movement disorders that are genetically inherited include dystonia musculorum deformans (a rare dystonia characterized by movements resulting in bizarre postures), Huntington's disease (results in chorea or uncontrolled movements, loss of intellectual faculties, and emotional disturbance), and Tourette's syndrome (multiple motor and vocal tics or repeated muscle contractions).
Sex: The sex of the individual predisposes certain people to movement disorders. In Tourette's syndrome, at-risk males are more likely to have tics and at-risk females are more likely to have obsessive-compulsive symptoms. Seven of every 10 girls who inherit the gene, and nearly all boys who inherit it, will develop symptoms of Tourette's syndrome. Huntington's disease affects both sexes equally.